Norovirus and mollusks: An explainer

oysters_blogIf there’s one thing to know about norovirus, it’s that it’s bad news.

Not only does the virus cause a person to be very ill (symptoms include “explosive” vomiting multiple times a day, along with diarrhea), it also is highly contagious. Although norovirus infection is most commonly associated with eating food contaminated by someone who is infected or direct transmission from someone with the virus, norovirus also can be transmitted by consuming raw mollusks.

The key lies in the way mollusks obtain food from their environment. Organisms such as clams, oysters and mussels feed by filtering water through their gills, which can also draw norovirus into their bodies. The amount of virus can become concentrated within the mollusk, often to higher levels than the surrounding water. And, once there, it’s almost impossible to extract, according to Public Health England.

However, before mollusks can filter norovirus-contaminated water through their gills, the virus has to be present in the water. This can happen in myriad ways, but most commonly it occurs when sewage or contaminated matter enters rivers and thereby contaminates shellfish beds.

Cooking can destroy the virus but frequently, mollusks are eaten raw (take oysters, for example). Likewise, the quick steaming methods often used to cook mollusks may be inadequate to knock out norovirus – as the Centers for Disease Control and Prevention (CDC) note, norovirus is a pretty hardy little bug and can withstand temperatures up to 140°F.

Cases of norovirus tend to jump in the colder months (in fact, it’s often referred to as the “winter vomiting bug”) as does detection of the virus in oysters. Norovirus also is one of the most common foodborne illnesses in the U.S. (about 58 percent of foodborne illness cases) and in Europe, foodborne viruses including norovirus are the second leading cause of foodborne illness.

Norovirus symptoms

Symptoms of norovirus typically last one to two days (or longer for those with comprised immune systems), and include diarrhea, “explosive” vomiting, headache, fever and chills. Less than one percent of cases require hospitalization, according to the U.S. Food and Drug Administration’s Bad Bug Book. Those with norovirus should not prepare or work with food. Infected surfaces should be cleaned thoroughly (see the CDC’s recommendations).

For more information on Neogen’s norovirus testing options, click here.

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